Note that the client side and server side SelectedIndexChanged events wont fire when the default item is selected (even if the AutoPostBack property is set to true). This is the default nature and these events will fire instantly once an item other than the default item in the RadComboBox is selected.
Please try the same control out of the RadListView and attach its event in the PageLoad server event and verify if it fires correctly.
Also sharing your page markup and full code behind will help us further investigate the issue and do our best to provide proper solution.
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